1. Field of the Invention
The present invention relates generally to a method and device for suppressing interference between the pilot channel and other channels in a code-division multiple access (CDMA) mobile telephone system.
2. Description of the Related Art
In accordance with Telecommunications Industry Association (TIA) standard IS-95 for CDMA telephone systems, each base station operates a pilot channel, a sync channel and forward traffic channels. These channels operate in the same frequency bandwidth and are made orthogonal to one another by the use of Walsh codes. Nevertheless, the orthogonality is degraded in a multipath environment and during soft handoff, and multipath components can interfere with one another. A non-orthogonal or noise contribution causes degradation in the pilot and traffic channel signals and the resulting output of the Rake detector. The pilot channel has the greatest likelihood of interfering with the other channels because it represents approximately 20 percent of total transmitted base station power. The pilot channel is an unmodulated, direct-sequence spread spectrum signal transmitted continuously by each CDMA base station. The pilot channel allows a mobile station to acquire the forward channel timing, provides a phase reference for coherent demodulation, and provides a basis for signal strength comparisons between base stations to facilitate soft handoff.
It has been suggested to cancel multipath noise contributed by the pilot signal. In one such system, a recursive method is used to reconstruct and subtract the multipath noise from the Rake finger inputs on the current symbol interval from the estimated channel coefficients on the previous symbol interval. A disadvantage of this system is that its circuit implementation is complex, computationally intensive, and uneconomical because the noise is canceled from the received signal before the signal is despread. It would be desirable to provide a system and method that suppresses interference caused by the pilot signal that is less complex and more economical than prior systems. The present invention addresses these problems and deficiencies in the manner described below.
The present invention relates to a system and method for canceling interference present in a code-division multiple access (CDMA) channel signal received at a CDMA receiver that is caused by multipath components of a transmitted pilot channel signal. The channel signal from which such multipath interference is canceled can be either a traffic (data) channel or the pilot channel itself. Some embodiments of the present invention can cancel interference from either the traffic channel or the pilot channel in accordance with a selector input signal. Although the terms xe2x80x9cpilot channelxe2x80x9d and xe2x80x9ctraffic channelxe2x80x9d are used herein for convenience because they are used in the IS-95 CDMA specification, it is understood that any analogous CDMA system in which an unmodulated channel and a data channel based upon the unmodulated channel are referred to by other names is within the scope of the present invention.
In a CDMA receiver that includes a conventional n-finger Rake receiver, the system includes n cancellation circuits, one corresponding to each finger. Each cancellation circuit (corresponding to one of the fingers) produces nxe2x88x921 interference signals, each corresponding to one of the other nxe2x88x921 fingers. In embodiments of the invention in which pilot channel interference is canceled from the traffic channel, the cancellation circuit (corresponding to one of the fingers) produces each of the nxe2x88x921 interference signals in response to the receiver""s locally generated traffic channel pseudonoise (pn) sequence corresponding to that finger and one of the receiver""s n locally generated pilot channel pn sequences corresponding to another one of the fingers. In embodiments of the invention in which pilot channel interference is canceled from the pilot channel itself, the cancellation circuit (corresponding to one of the fingers) produces each of the interference signals in response to the receiver""s locally generated pilot channel pn sequence corresponding to that finger and one of the receiver""s n locally generated pilot channel pn sequences corresponding to another one of the fingers. In an exemplary embodiment, the cancellation circuit interpolates the locally generated pilot channel pn sequence corresponding to another finger before using into generate intermediate samples between pn chips. Also, in the exemplary embodiment, the interpolator is a digital filter having an impulse response approximating a convolution of the output filter of the transmitter and the input filter of the receiver.
Each cancellation circuit also produces nxe2x88x921 correction signals. The cancellation circuit (corresponding to one of the fingers) produces each of the nxe2x88x921 correction signals in response to the interference signal corresponding to another one of the fingers and the received pilot channel signal corresponding to that other finger. The received pilot signal of the other finger represents an estimate of the gain and phase of the multipath pilot signal to be canceled.
The system also includes n correction circuits, one corresponding to each finger. In embodiments of the invention in which pilot channel interference is canceled from the traffic channel, the correction circuit (corresponding to one of the fingers) subtracts each of the nxe2x88x921 correction signals corresponding to that finger from the received traffic channel signal corresponding to that finger. In embodiments of the invention in which pilot channel interference is canceled from the pilot channel itself, the correction circuit (corresponding to one of the fingers) subtracts each of the nxe2x88x921 correction signals corresponding to that finger from the received pilot channel signal corresponding to that finger.
The present invention is more efficient, economical and consumes less power than prior cancellation systems because, by subtracting the interfering multipath pilot signal components from the received traffic or pilot signal, i.e., after the receiver has despread it, the cancellation and correction circuits can operate at symbol rates rather than chip rates.